East German Patent 117,580 discloses a process for preparing a fluorinated alumina catalyst in the presence of promoter metals from an activated alumina, and its use for fluorinating two-carbon compounds in the gas phase. Asymmetrical C.sub.2 fluorocarbons and pentafluorochloroethane (FC-115) are claimed to be prepared by this process using a molar ratio of tetrachloroethylene/chlorine/hydrogen fluoride of 1/1/3 to 1/1/5.1. Pentafluorochloroethane concentrations ranged from 6% to 57% of the products at 350.degree. C. and 8.5 s contact time, and at 450.degree. C. and 7.3 s contact time, respectively.
Vechhio, M. et. al., J. Fluorine Chem., 4, 117-39 (1974) describe the chlorofluorination of tetrachloroethylene using Cl.sub.2 and HF at ca. 400.degree. C. in the presence of an aluminum fluoride catalyst to yield the asymmetrical isomers of dichlorotetrafluoroethane and trichlorotrifluoroethane as the major products. In
another experiment a 1/1 molar mixture of CCl.sub.2 FCClF.sub.2 (FC-113)/HF was passed over an AlF.sub.3 catalyst at 400.degree. C., and 10.3 mol % FC-115 and 2.5 mol % C.sub.2 Cl.sub.4 F.sub.2 (FC-112 and FC112a) were obtained. In a comparable experiment using a 1/1 FC-113/N.sub.2 molar mixture, 6.8 mol % FC-115 and 8.8 mol % C.sub.2 Cl.sub.4 F.sub.2 (FC-112 and FC-112a) were observed. The FC-115 concentration is dependent on both the HF concentration and disproportionation reactions, while the FC-112s concentration is dependent only on disproportionation reactions. The FC-112s concentration when HF was in the feed suggest that disproportionation reactions are occurring to a significant extent when the HF/FC-113 molar ratio is 1/1; additionally, more FC-115 was observed with HF in the feed.
U.S. Pat. No. 4,605,798 discloses a process for the preparation of trichlorotrifluoroethane, dichlorotetrafluoroethane, and monochloropentafluoroethane by the reaction of chlorine, HF, and tetrachloroethylene over a variety of catalysts, some of which are claimed to be oxides or halides of aluminum. FC-115 is disclosed to be produced in a fluorination-dismutation (i.e., disproportionation) zone by the reactions of HF and dichlorotetrafluoroethane. Furthermore, the amount of asymmetrical isomer CF.sub.3 CCl.sub.2 F (FC-114a) produced is claimed to comprise less than 7% of the dichlorotetrafluoroethane products
GB 1,578,933 claims a process for the preparation of 1,1,1,2-tetrafluoroethane (FC-134a) or 1,1,2,2-tetrafluoroethane (FC-134) by hydrogenating a haloethane having four or five fluorine atoms of formula CF.sub.2 XCFYZ where X is fluorine or chlorine; and when X is fluorine, Y is chlorine or fluorine; and when Y is chlorine Z is chlorine, fluorine or hydrogen; and when Y is fluorine Z is hydrogen; and when X is chlorine Y is fluorine and Z is either chlorine or hydrogen, over a supported Pd catalyst at a temperature of 200.degree. C. to 450.degree. C. In example 7 it is seen that in order to convert all of the intermediate 1,1,1,2-tetrafluoro-2-chloroethane (FC-124) to FC-134a a hydrogenolysis temperature of greater than 350.degree. C. is required.
Gervasutti, C. et. al., J. Fluorine Chem., 19, 1-20 (1981) disclose a process for the preparation of FC-134a from isomeric mixtures of dichlorotetrafluoroethanes through selective hydrogenolysis of 1,1,1,2-tetrafluoro-2,2-dichloroethane (FC-114a) catalyzed by Pd/C. The 1,1,2,2-tetrafluoro-1,2-dichloroethane (FC-114) was more stable toward hydrogenolysis. The concentration of FC-134a, from FC-114a hydrogenolysis, in the reaction products is shown to increase up to a maximum temperature of about 200.degree. C. above which its concentration decreases.
This invention provides a multi-step process with recycle for preparing 1,1,1,2-tetrafluoroethane (FC-134a), useful as a refrigerant, in very high yield by minimizing the formation of CF.sub.3 CClF.sub.2 (FC-115) via disproportionation of C.sub.2 Cl.sub.2 F.sub.4 (FC-114 and FC-114a), without suppressing the isomerization of CClF.sub.2 CClF.sub.2 (FC-114) to CF.sub.3 CCl.sub.2 F (FC-114a).